a) Field of the invention
This invention relates to a stereomicroscope.
b) Description of the prior art
Stereomicroscopes are broadly classified into two systems: a Greenough type stereomicroscope provided with two observing optical systems whose optical axes are bisymmetrically arranged at a predetermined angle (which will be hereinafter referred to as an internal inclination angle) to observe an object with eyes and a Galileo type stereomicroscope provided with a single objective lens, used in common, arranged so that afocal images are formed by emanated beams of light and two observing optical systems arranged bisymmetrically, in parallel with each other, behind the single objective lens to observe the images with eyes.
FIG. 1 shows the optical system of the Greenough type stereomicroscope. In this figure, reference symbols V.sub.L and V.sub.R designate optical axes of observing optical systems disposed on the left and right sides, respectively; .theta. an internal inclination angle; numerals 1 and 1' first lens components on the left and right sides, respectively; 2 an object surface; 3 and 3' second lens components on the left and right sides, respectively; 4 and 4' image rotator prisms, on the left and right sides, changing real images formed by the second lens components 3, 3' to orthographic images, respectively; and 5 and 5' eyepieces, on the left and right sides, respectively, for magnifying and observing the orthographic real images.
FIG. 2 depicts the optical system of the Galileo type stereomicroscope. In this figure, reference symbols V.sub.L and V.sub.R represent the optical axes of the observing optical systems arranged on the left and right sides, respectively; numeral 2 the object surface; 6 a single objective lens used in common; 7 and 7' imaging lenses, on the left and right sides, respectively, changing afocal images formed by the single objective lens to real images; 4 and 4' the image rotator prisms, on the left and right sides, respectively, changing the real images to orthographic images; and 5 and 5' the eyepieces on the left and right sides, respectively.
Although the Galileo type stereomicroscope has the defect that an image plane is viewed to swell up in a convex shape as will be described later, such a defect is obviated by an improved stereomicroscope of Japanese Patent Preliminary Publication No. Sho 61-39017.
The Greenough type stereomicroscope, on the one hand, has possessed the defect that flatness of an image is poor on principle because the object surface is not perpendicular to each of the optical axes on the left and right sides. This respect will be described in detail as follows:
FIG. 3 is an enlarged view of a range M of FIG. 1 and shows the relationship between the depth of field and the internal inclination angle in the object surface 2. In this figure, reference symbol 0 represents a center of a visual field, that is, an intersection of the optical axis V.sub.L with the optical axis V.sub.R ; L.sub.1 a straight line where a plane including the optical axes V.sub.L, V.sub.R intersects with the object surface 2; L.sub.2 a straight line where the plane including the optical axes V.sub.L, V.sub.R intersects with a plane including the point 0, normal to the optical axis V.sub.L ; A and B ends of the visual field on the straight line L.sub.1 (that is, the length of a segment AB corresponds to the range of the visual field); .DELTA.d a depth of field on an object side in the observing optical system on the left side; Na and Nb straight lines, parallel to the straight line L.sub.2, at the ends of the range of the depth of field; and C and D intersections of the straight lines Na and Nb with the straight line L.sub.1, respectively.
As is obvious from FIG. 3, in the observing optical system on the left side, observation can be made without blur between the lines Na and Nb, namely, in the range of the depth of field .DELTA.d. Thus, on the line L.sub.1, a segment between the points C and D is the range in which observation can be made without any blur. Here, because the range of the visual field lies between the points A and B, the flatness of the image in such an instance comes to CD/AB and is unlikely to become larger, that is, better.
Also, for the intention of bringing about high resolution, it is only necessary that the aperture of each of the first lens components of the observing optical systems on the left and right sides is increased to secure a larger numerical aperture (N. A.). However, since the first lens components are arranged at the internal inclination angle, such a situation that the first lens components on the left and right sides come into contact with each other is regarded as the upper limit of the size of the apertures of the first lens components. Accordingly, this stereomicroscopic system will also be limited in resolution. In addition to this, the system has the disadvantage inconsistent with the principle that the depth of field decreases as the numerical aperture is made larger and consequently the flatness of the image deteriorates. Specifically, since the relationship between them is such that the depth of field .DELTA.d is inversely proportional to the square of the numerical aperture of the observing optical system, the depth of field .DELTA.d decreases rapidly as the numerical aperture is made larger, that is, the resolution higher, and the value of the segment CD also diminishes. In other words, the flatness of the image will grow worse.
On the other hand, the Galileo type stereomicroscope has had the drawback that since the use of the single objective lens 6 with a low manufacturing cost composed of one to three lenses brings about the generation of asymmetric distortion onto the afocal image derived from the single objective lens 6, a plane-shaped specimen is not observed as a plane as it is, but viewed to swell up in a convex shape.
Although this drawback is obviated by making use of the single objective lens of the stereomicroscope set forth in the Sho 61-39017, problems have been encountered that the number of lenses constituted in such a case increases with resultant high manufacturing costs.
This respect will be mentioned in detail as follows:
FIG. 4 is cited from FIG. 2 of the Sho 61-39017. In this figure reference symbols V.sub.L and V.sub.R designate the optical axes of the observing optical systems disposed on the left and right sides, respectively; numeral 6 the single objective lens; 2 the object surface; 7 and 7' the imaging lens on the left and right sides, respectively; symbol P a given point on a Y axis included in the object surface 2; V.sub.1 and V.sub.2 rays of light emanating from the point P to pass through the centers of pupils of the observing optical systems on the left and right sides, respectively; and .theta..sub.L, and .theta..sub.R, angles made by the light rays V.sub.1, V.sub.2 with the optical axes V.sub.L, V.sub.R, respectively, immediately after the light rays V.sub.1, V.sub.2 emerge from the single objective lens 6. According to the Sho 61-39017, in order that the image formed by the single objective lens 6 is devoid of the asymmetric distortion and the planate specimen is observed as a plane as it is, there are indications that the angles .theta..sub.L and .theta..sub.R will profit from the use of the single objective lens 6 such as to satisfy the following equation: ##EQU1##
However, as seen from FIGS. 5A and 5B cited from the views of Embodiments 1 and 3, respectively, of the Sho 61-39017, a lens configuration such that the angles .theta..sub.L and .theta..sub.R shown in FIG. 4 are to satisfy the equation: ##EQU2## in the Galileo type stereomicroscope will need more than four lenses constituting the objective lens 6 and will be rendered considerably complicated.